The need for repeatable accurate chemical analysis methods and apparatus is ever increasing. In response to this need, a variety of analyzers have been built. With each new analyzer, the focus has consistently been on the construction of analysis apparatus which increases analysis apparatus capacity and reduces the number of required steps in the analysis process. Flow injection analyzers have been built to meet these needs.
Flow injection analyzers are instruments capable of detecting features of a sample injected into a continuously flowing solution. Flow injection analysis is based on an analysis system capable of forming a reproducible gradient of sample in a reagent flow, detectable as a gradient curve. Measurements carried out on the resultant gradient curve are used to determine the characteristics and components of the sample.
A new area in the field of flow injection analysis is flow injection titrimetry (F.I.T.) which combines the best features of flow injection analysis with titrimetry techniques.
Flow injection titrimetry is derived from titration which is the volumetric determination of a constituent in a known volume of a solution by the slow addition of a standard reacting solution of a known strength until the reaction is completed. Completion of the reaction is frequently indicated by a color change (indicator) or electrochemical change in the solution.
Flow injection titrimetry (F.I.T.) has been developed to produce rapid, simple, reliable, versatile and accurate analysis systems for process control applications. Different from other flow injection analysis techniques, flow injection titrimetry is based on the measurements of peak width rather than peak height. The width of this peak is proportional to the log of the sample concentration. Contrary to other flow injection analysis techniques, flow injection titrimetry makes use of a large sample dispersion to create a concentration gradient over time. This concentration gradient is known as the "exponential concentration gradient". The exponential concentration gradient is the concentration gradient within the mixing cell during flow injection analysis.
The concept of single point titration using flow injection analysis techniques has been described for acid/base systems by Ove .ANG.strom's article, "Single-Point Titrations" found in Analytica Chimica Acta, 105 (1979) 67-75. The .ANG.strom's method for a single-point titrimetric system for acids and bases utilizes a reaction cell consisting of a reference electrode, a glass electrode, a mixing coil 300 cm long, and teflon injection tubing. Only one analysis can be performed using the reaction cell with detection electrodes. A need has long been felt for a dual analysis system.
Multielement trace analyzers using nonsegmented continuous flow analysis have been described in "Correspondence", Analytical Chemistry, Vol. 50, No. 4, (1978) 654-656. However, this analysis technique is taught in only a very general way. The multielement trace analysis using nonsegmented continuous flow for the compounds 4-(2-pyridylazo) resorcinol (PAR), lead (II) and vanadium (V) is colorimetric rather than titrimetric. No specific teaching of multielement trace analysis using flow injection titrimetry has been found, particularly for caustic/carbonate systems.
The apparatus used in multielement trace analysis, generally has included a reaction cell, a measuring instrument, and a recorder or data processing unit, see, "Injection Technique In Dynamic Flow-Through Anaylsis With Electroanalytical Sensors" by Pungor, Feher, Nagy, Toth, Horvai and Gratzl, appearing in Analytica Chemica Acta, 109 (1979), 1-24. This apparatus has not been capable of both acting as a reaction cell and a detection cell for multiple endpoint flow injection titrimetry. The present invention seeks to provide such an instrument and an accompanying flow injection titrimetry technique.
Known analysis methods have utilized batch analysis methods for detecting endpoints of independently titratable species in caustic/carbonate reactions. One batch technique, as described in Scotts', Standard Methods of Chemical Analysis (5th Ed., p. 2256) describes a double-endpoint determination of sodium hydroxide and sodium carbonate in a mixture thereof by (a) titrating with sulfuric acid to the phenolphthalein endpoint (NaOH converted to NaHS0.sub.4 and H.sub.2 O; Na.sub.2 CO.sub.3 converted to NaHCO.sub.3) and (b) titrating further with sulfuric acid to the methyl-orange endpoint (NaHCO.sub.3 converted to NaHSO.sub.4, CO.sub.2 and H.sub.2 O). However, batch techniques have numerous drawbacks since they are not capable of continuous quantitative measurements nor continuous titration analysis. The batch titrations must be periodically stopped and the reactors must be cleaned after each reaction is completd. This known technique has required abundant analysis time to obtain the necessary results. A need has existed for determining multiple end points of independently titratable species in a continuous flow, nonbatch type of titration system.
Known continuous flow injection analysis techniques have been developed for continuous flow acid-base titration as described in J. Ruzicka, and E. H. Hansen, Flow Injection Analysis, Wiley-Interscience Publication, (Chemical Analysis, Vol. 62), 1981. With the batch tank model developed by Ruzicka et al., the time span between these two observed equivalence points, t.sub.eq, may be expressed by the following equation: ##EQU1## where V.sub.m is the mixing cell volume which is much larger than the sample volume, V.sub.s, C.sub.so is the concentration of S in the mixing cell at t=0, Q.sub.s is the sample flow rate, and Q.sub.t is the titrant flow rate and thus ##EQU2## where C.sub.s @ is the initial concentration of S. With a single channel manifold as used in this work, EQU Q.sub.s =Q.sub.t =Q (3)
and the above equation reduces to ##EQU3## where C.sub.t is the concentration of the titrant. Therefore, for the titration of base with an acid ##EQU4## where n is the number of equivalents weight of the acid. Rearranging this equation and substituting the equation for C.sub.so, a linear equation is obtained with the form of EQU ln.sub.e C.sub.base =K.sub.l t.sub.eq +k.sub.2. (6)
The slope of the response curve is affected by V.sub.m and Q. The intercept, and thus the lower limit of detection, is affected by V.sub.s, V.sub.m, and C.sub.acid. Thus if V.sub.s and V.sub.m are kept constant, the sensitivity of the method can be changed by varying Q; and the lower limit of detection can be changed by varying C.sub.acid. Flow injection titrimetry methods are capable of providing detection limit sensitivities which can be chosen to fit the needs of the analyst.
A problem with the above described single channel system is that the results were limited to the analysis of one component, that is, where C.sub.s is the molar concentration of species to be titrated; K is the constant related to the apparatus including cell volume and flow rate; teq is the time to an equivalence point, i.e., t.sub.i ; C is the constant relating concentration of the titrant; V.sub.m is the volume in the mixing cell; and Q is the flow rate then: ##EQU5## Ruzicka and Hansen also developed another titration system, described in "Recent Developments in Flow Injection Analysis: Gradient Techniques and Hydrodynamic Injection", Analytica Chimica Acta, 145 (1983), 1-15. However, this continuous flow multiple endpoint titration system is limited to a teaching for a single component acid-base titration. In particular, the authors focus on the titration of phosphoric acid by 1.times.10.sup.-3 M sodium hydroxide, and do not address a multiple component flow injection analysis multiple endpoint system.
Yet another flow injection titrimetry technique was taught in U.S. Pat. No. 4,283,201 to DeFord et al. In that reference, a titrant is supplied to two parallel fluidly communicating circuits. The first circuit involved a pressure regulator means and a flow restricting means terminating in a first electrical conductivity detection cell means having a vent means. The second circuit involved a flow rate controller means, a sample valve means and a chromatograph column or equivalent means terminating in a second electrical conductivity detection cell means also having a vent means. The electrical output signals representing the electrical conductivity of the fluid conducted through the first cell means and the second cell means were combined in an electrical difference detection means. The electrical output signal generated within the detection means and representative of the difference in the tw fluid conductivities was passed to one channel of a dual channel strip chart recording means and additionally passed to an electrical signal derivative detection means. The electrical output signal, representative of the derivative of said difference signal, was then passed to a digital clock and counter means and then to a recording means. The material or sample to be reacted or titrated was supplied to the two parallel fluidly communicating circuits from a third conduit.
The DeFord teaching provided a method and apparatus for flow injection titrimetry which used a plurality of reactant streams, analyzers, and detection apparatus to detect a plurality of end points of a complex sample. This teaching has not satisfied all the needs of the medical, pharmaceutical and argicultural fields in regard to analysis apparatus. A need exists for a flow injection method of analysis which provides data regarding a plurality of end points requiring less equipment and less time than the DeFord teaching. A method and device have long been needed for performing multiple endpoint titrations in a single analysis. The present invention seeks to go beyond these teachings and present a method of nonlinear multiple endpoint flow injection titrimetry for several species of sample.
One problem with prior mixing cells for titration flow injection analysis is entrapment of bubbles in the mixing cell. This problem is related to the mixing action of the stirrer in the mixing cell. Bubbles tend to collect on cell walls and to remain in the vortex of the stirred contents of the cell and interfere with analytical accuracy. Special stirrers such as the Fisher Scientific stir bar for spectrophotometer cells, Catalog No. 14-511-72, are designed to minimize aeration and are effective in spectrophotometric cuvettes. They are, however, inadequate in Flow Injection titration analysis because of a demonstrated tendency to form and trap bubbles on the stirrer itself and on the tip of the detector probe. The trapped bubbles interfere with the analytical accuracy. The stirrer of the present invention results in helical flow of the carrier from the inlet to the outlet of the mixing cell with a minimum of vertical mixing (assuming vertical progression of helical flow in the mixing cell). This helical flow, preferably in combination with a chamber which is narrowed near the outlet of the chamber to allow bubble coalescing which facilitates bubble removal from the cell and effectively solves the above mentioned bubble problem.